The present invention relates to a curable resin capable of providing a cured product having excellent heat resistance, moisture resistance, and flexibility, and a curable resin composition containing the curable resin.
Epoxy acrylate (i.e., a vinyl ester resin), which is produced by modifying an epoxy resin with an unsaturated monobasic acid, can be cured with heat or light, and its chemical resistance is excellent. Due to such advantages, epoxy acrylate is used as a curable resin in applications such as various molding materials and coating materials.
However, in these applications, there is a problem that the epoxy acrylate causes the decrease in the free volume of the molding material and the coating material in the curing process. The decrease in the free volume generates the accumulation of internal stress. As a consequence, the molding materials form molded products which has cracks and warping, and is poor in properties such as heat resistance and moisture resistance. In addition, the coating materials form coating layers having poor adhesion to substrates to be coated.
In addition to the above applications, the epoxy acrylate is widely used as a photo-curable resin for image formation and minute work utilized for patterning a printed circuit board. In this industrial field, there is a demand for a resin to which a principle of photo-imaging can be applied as the minuteness of the printed circuit board proceeds, and also which can be developed in a diluted weak alkaline aqueous solution for environmental protection. From these points of view, at the present, epoxy acrylate containing carboxyl groups has been used as a photo-curable resin for image formation. The carboxyl group-containing epoxy acrylate is obtained by reacting epoxy acrylate with polybasic anhydride to introduce carboxyl groups (for example, Japanese Unexamined Patent Publications Nos. 61-243869 and 63-258975).
In the formation of patterns using the above-described liquid photo-curable resin composition containing the above-described photo-curable resin, the following process is employed. First, the curable resin composition is applied onto the surface of substrate, and then, is heated and dried to form a coating layer. Onto the coating layer, a patterning film is compressed, the resultant is exposed to light, then, is developed. In this process, if the coating layer is tacky after the heating and drying step, there arises a problem that, when the patterning film is peeled off, a part of resin composition adheres to the patterning film and patterns cannot be correctly reproduced, or the patterning film cannot be peeled off. Accordingly, the ability of forming the coating layer with tack-free property is an important requirement for the liquid photo-curable resin composition.
Furthermore, the developability after exposure to light is also an important requirement. That is, in order to form fine patterns with a high reliability at a good reproducibility, unexposed portions of the coating layer have to be removed rapidly in the developing step. However, the developability and the tack-free property are inconsistent characteristics with each other. The improvement of the developability tends to impair the tack-free property. There is a difficulty in attaining both good developability and good tack-free property.
Furthermore, the photo-curable resin has the same problem that the curable resin used in the molding material has. That is, the coating layer after pattern formation is subjected to high temperature or high moisture, it cracks or peels off from the substrate.
In the above mentioned problems, there has been proposed an idea to achieve the improvement of the developability, that the hydroxyl group is introduced to the resin with the reaction between epoxy- group and carboxyl group (Japanese Unexamined Patent Publication No. 7-242716). However the obtained cured resin was insufficient in resistance under the condition of high temperature and humidity. Under such circumstances, there is a demand for developing a photo-curable resin satisfying all the important characteristics including tack-free property, developability, adhesion to substrate, and the like.